The justices decided that the wording of the Armed Career Criminal Act that defines what crimes make a defendant eligible for extremely long prison sentences.
The case was appealed by Samuel James Johnson, who pleaded guilty to federal weapons charges in 2012, yet he was sentenced to 15 years in prison. 15 years is five more than the designated sentence for such a crime, yet due to prior convictions he was sentenced to a longer prison term.
The court agreed to hear Johnson's case to decide whether his prior conviction for possessing a sawed-off shotgun qualified as a violent felony under the lengthier sentencing law. The law lists arson, burglary, extortion and the use of explosives as crimes that can lead to a longer sentence. The law also pertains to cases that “otherwise involves conduct that presents a serious potential risk of physical injury to another.” Six justices agreed that the phrase is unconstitutional. Justice Scalia said, “[language] so shapeless a provision to condemn someone to prison for 15 years to life” violates the Constitution's guarantee of due process.
The Armed Career Criminal Act makes defendants entitled to longer prison terms if they have three prior convictions for crimes that are either violent felonies or serious drug offenses.
You can read the Supreme Court Decision here: Supreme Court Strikes Down Career Criminal Act